Explosive detection in many environments, such as military facilities, minefields, remediation sites, as well as mass transit areas has unfortunately become a necessity. Chemical sensors have shown a high aptitude for the detection of trace levels of explosives. For that reason, there remains a demand for inexpensive and reliable explosive sensors to be used in a field setting for identifying specific explosives.
The majority of chemical sensors are based upon small synthetic molecules that produce a measurable signal upon interaction with a specific analyte. Chemical sensors are cost effective and have been shown to succeed where other techniques fail to effectively detect explosives. For example, modern land mines are encased in plastic and can be easily missed by metal detectors. Trained explosive sniffing dogs are effective, but require extensive, expensive training and can be difficult to maintain. Other detection methods, such as gas chromatography coupled with a mass spectrometer, surface-enhanced Raman, nuclear quadrupole resonance, energy-dispersive X-ray diffraction, neutron activation analysis and electron capture detection are highly selective. These methods are expensive and from not easily transported systems that are difficult to deploy and utilize in a field setting.
There are many commercially available chemical sensors that function very well under many circumstances, however all have limitations that render them ineffective under some conditions. The detection of TNT (2,4,6-trinitrotoluene) and picric acid in groundwater or seawater can be critical for the detection of buried, undetonated ordinances or for locating underwater mines. This can be problematic for chemical sensors because most chemical sensing detection methods are optimized for air samples. Interference problems are encountered in complex aqueous media. Standard chemical sensors can therefore be inefficient in environmental applications for characterizing soil and groundwater contaminated with toxic explosives at military bases and munitions production and distribution facilities.
Conventional chemical sensors, such as highly π-conjugated, porous organic polymers, can be used to detect vapors of electron deficient chemicals, but require many steps to synthesize, are not always selective to explosives and cannot be considered a cost effective disposable sensor. The problem of vapor detection is further hampered when the vapor pressure of explosives is considered.
Organic and silicon metallole hybrid polymers developed by the present inventors and colleagues have shown the ability to detect explosives by way of fluorescent quenching even in complex media. For example, Sailor et al., U.S. Pat. No. 7,482,168 entitled Photoluminescent Polymetalloles as Chemical Sensors, discloses methods for detecting electron deficient nitroaromatic molecules in air, water or other surfaces that employ a thin film of photoluminescent copolymers, which are stable in air, water, acids, common organic solvents, and even seawater containing bioorganisms. The polymers contain metalloid-metalloid backbones such as Si—Si, Si—Ge, or Ge—Ge. The detection method involves measurement of the quenching of photoluminescence of the polysilole by the analyte. Trogler et al., U.S. Pat. No. 7,927,881 entitled Inorganic Polymers and Use of Inorganic Polymers for Detecting Nitroaromatic Compounds, discloses sensing methods and sensors including an inorganic-organic metallole-containing polymer or copolymer with a backbone including carbon atoms bonded to metalloid atoms. Additional sensors and sensing methods are disclosed in Sohn et al., “Detection of Nitroaromatic Explosives Based on Photoluminescent Polymers Containing Metalloles,” J. Am. Chem. Soc., 2003, 125 (13) pp. 3821-30 (2003); Sohn et al., “Detection of TNT and Picric Acid on Surfaces and in Seawater by Using Photoluminescent Polysiloles,” Angew. Chem. Int., vol. 40, No. 11, pp. 2104-2105 (2001).
Most high explosives are organic nitrates, nitramines, and nitro based compounds. High explosives are considered to be organic and oxidizing, a relatively rare combination that makes them tractable for molecular recognition-binding event involving hydrophobic interactions, followed by electron transfer quenching of a fluorophore. The Lewis acid nature of the Si atom also permits coordination of oxygen lone pairs from the analyte, providing an efficient electron transfer pathway. For this reason, fluorescent silole hybrid polymers have had favorable success in their use as high explosive sensors.
Fluorescence quenching polymers selective for nitroaromatics have been demonstrated to allow for separation and identification of nitroaromatic explosives, such as TNT, DNT, tetryl, and TNB through a combination of fluorescence quenching and chromatographic retention time. For other high explosives (e.g., PETN, RDX, HMX) a fluorescent polymer capable of detecting these explosives is used if the sample is negative for nitroaromatics. Through a combination of fluorescence quenching and by the characteristic chromatographic retention time, the high explosive can be identified. In typical prior sensing methods, this class of organosilicon polymers have been dissolved and sprayed onto solid supports as aerosols to provide a sensor.
Decades ago, Sawicki, et al. pioneered the use of TLC separation and fluorescence quenching in the identification of over 20 different polyaromatic hydrocarbon (PAH) compounds present in environmental combustion aerosols. E. to Sawicki et al., “Direct Fluorometric Scanning of Thin-layer Chromatograms and its Application to Air Pollution Studies,” Journal of Chromatography Vol. 20, lines 348-353. These methods rely on quenching the fluorescence of the PAH itself with organic nitro compounds, and detection limits as low as 10 ng were achieved. Colorimetric reagents have been used previously to further identify PAH compounds, including TNT, with the use of TLC and paper chromatography, but such methods are limited to detection limits of microgram quantities. Thin layer chromatography in conjunction with optical absorption measurements has been used to separate explosives and identify compositions of explosive mixtures, but did not provide for trace detection. See, e.g., “Determination of RDX and ket9-RDX in High-explosive Mixtures by High-performance Thin-Layer Chromatography,” J. Planar Chrmmatog, 14 (2001) 296-299.